"A transitional governing body happens where the state is in disintegration, or has no institutions."

The peace conference almost collapsed yesterday, the day face-to-face talks were meant to start, and was only put back on track after Lakhdar Brahimi persuaded the two sides to focus on smaller issues on which there might be agreement.

"We do expect some bumps on the road," Mr Brahimi told a news conference after separate meetings with the two delegations.

With international divisions over how to end the conflict putting an overall political solution out of reach for now, the two sides will focus on small, confidence-building steps with no certainty negotiations will even last the week.

Humanitarian access for Homs, where rebels are surrounded in central districts by forces loyal to President Bashar al-Assad, could be agreed quickly.

"The practical aspects have been worked on. Things are ready and if the government doesn't put a block on it then it could happen quickly," a diplomatic source said.

But deep mutual mistrust and the absence from Geneva of powerful Islamist opposition groups and Mr Assad's ally Iran make any substantial progress very difficult.

The opposition said early on Friday it would not meet the government side unless it first agreed to publicly endorse a 2012 statement by world powers calling for a transitional government in Syria.

The government rejected the demand and said its negotiators would leave Geneva unless serious talks began within a day.